Materials such as silicon (Si) and gallium arsenide (GaAs) have found wide application in semiconductor devices. These more familiar semiconductor materials may not be well suited for higher power and/or high frequency applications, however, because of their relatively small bandgaps (e.g., 1.12 eV for Si and 1.42 for GaAs at room temperature) and/or relatively small breakdown voltages.
Accordingly, interest in high power, high temperature and/or high frequency applications and devices has turned to wide bandgap semiconductor materials such as silicon carbide (2.996 eV for alpha SiC at room temperature) and the Group III nitrides (e.g., 3.36 eV for GaN at room temperature). These materials typically have higher electric field breakdown strengths and higher electron saturation velocities as compared to gallium arsenide and silicon.
A device of particular interest for high power and/or high frequency applications is the High Electron Mobility Transistor (HEMT), which, in certain cases, is also known as a modulation doped field effect transistor (MODFET). These devices may offer operational advantages under a number of circumstances because a two-dimensional electron gas (2DEG) is formed at the heterojunction of two semiconductor materials with different bandgap energies, and where the smaller bandgap material has a higher electron affinity. The 2DEG is an accumulation layer in the undoped (“unintentionally doped”), smaller bandgap material and can contain a very high sheet electron concentration in excess of, for example, 1013 carriers/cm2. Additionally, electrons that originate in the wider-bandgap semiconductor transfer to the 2DEG, allowing a high electron mobility due to reduced ionized impurity scattering.
This combination of high carrier concentration and high carrier mobility can give the HEMT a very large transconductance and may provide a strong performance advantage over metal-semiconductor field effect transistors (MESFETs) for high-frequency applications.
High electron mobility transistors fabricated in the gallium nitride/aluminum gallium nitride (GaN/AlGaN) material system have the potential to generate large amounts of RF power because of the combination of material characteristics that includes the aforementioned high breakdown fields, their wide bandgaps, large conduction band offset, and/or high saturated electron drift velocity. A major portion of the electrons in the 2DEG is attributed to polarization in the AlGaN.
HEMTs in the GaN/AlGaN system have already been demonstrated. U.S. Pat. Nos. 5,192,987 and 5,296,395 describe AlGaN/GaN HEMT structures and methods of manufacture. U.S. Pat. No. 6,316,793, to Sheppard et al., which is commonly assigned and is incorporated herein by reference, describes a HEMT device having a semi-insulating silicon carbide substrate, an aluminum nitride buffer layer on the substrate, an insulating gallium nitride layer on the buffer layer, an aluminum gallium nitride barrier layer on the gallium nitride layer, and a passivation layer on the aluminum gallium nitride active structure.
Wide bandgap GaN-based high-electron-mobility-transistors (HEMTs) have come a long way as microwave devices since their description in 1993 in Khan et al., Appl. Phys. Lett., vol. 63, p. 1214, 1993, and a demonstration of their power capability in 1996 in Wu et al., IEEE Electron Device Lett., vol. 17, pp. 455-457, September 1996. Many research groups have presented devices with power densities exceeding 10 W/mm, a ten-fold improvement over conventional III-V devices. See Tilak et al., IEEE Electron Device Lett., vol. 22, pp. 504-506, November 2001; Wu et al., IEDM Tech Dig., Dec. 2-5, 2001, pp. 378-380; and Ando et al., IEEE Electron Device Lett., vol. 24, pp. 289-291, May 2003. An overlapping gate structure, or field plate, was used by Zhang et al. with GaN HEMTs for high-voltage switching applications. Zhang et al., IEEE Electron Device Lett., vol. 21, pp. 421-423, September 2000. Following this, Karmalkar et al. performed simulations for the field plate structure, predicting up to five times enhancement in breakdown voltages. Karmalkar et al., IEEE Trans. Electron Devices, vol. 48, pp. 1515-1521, August 2001. However, fabricated devices at that time had low cutoff frequencies, not suitable for microwave operation. Ando et al. recently used a similar structure with smaller gate dimensions and demonstrated performance of 10.3 W output power at 2 GHz using a 1-mm-wide device on a SiC substrate. Ando et al., IEEE Electron Device Lett., vol. 24, pp. 289-291, May 2003. Chini et al. implemented a new variation of the field-plate design with further reduced gate dimensions and obtained 12 W/mm at 4 GHz from a 150-μm-wide device on a sapphire substrate. Chini et al., IEEE Electron Device Lett., vol. 25, No. 5, pp. 229-231, May 2004.
Recently, GaN-based HEMTs with electric field modification by field plates have boosted power density to greater than 30 W/mm at frequencies up to 8 GHz. See, e.g., Y.-F. Wu et al., “30-W/mm GaN HEMTs by Field Plate Optimization,” IEEE Electron Device Lett., Vol. 25, No. 3, pp. 117-119, March 2004. However, since a field plate adds parasitic capacitances to a device and reduces gain, the design and fabrication of a GaN based HEMT capable of millimeter wave operation has been difficult.
Millimeter wave transistor operation (i.e. transistor operation at frequencies exceeding about 30 GHz) presents additional challenges due to the switching speeds required for such frequencies. Notwithstanding these challenges, millimeter wave devices have been demonstrated in gallium arsenide (GaAs) technology. In particular, GaAs pHEMTs have achieved 8.5 W output from devices having a gate periphery of 14.7-mm during near-millimeter wave operation. See, e.g., M. R. Lyons, et al., IEEE MTT-S/IMS Proceedings, pp. 1673-1676, Fort Worth, Tex., Jun. 6-11, 2004. However, for a given level of power output at a desired frequency of operation, GaAs devices tend to be much larger than corresponding GaN-based devices.
Applications for millimeter wave devices include digital radio transceivers for cellular communications backhaul, and ground terminal transceivers for very small aperture terminals (VSATs). Such devices may operate within radio bands up to 42 GHz, including frequencies in the Ku (12 GHz to 18 GHz) and Ka (26 GHz to 40 GHz) frequency bands. Additional applications exist in E-band (60 GHz to 90 GHz) of millimeter wave frequencies.